50 free Eris trivia questions with answers — science & nature quiz, new questions added Aug 2026.
This Eris trivia quiz covers the distant dwarf planet whose discovery cost Pluto its planethood. It starts with the find: Mike Brown's Palomar team, images from 2003 that software had ignored because Eris moved too slowly, NASA's 'tenth planet' announcement, the Xena and Gabrielle nicknames, and the long wait for a real name while the IAU decided what a planet was. Then the naming itself: Persephone rejected, 'Lila' abandoned, the goddess of strife adopted, and a moon called lawlessness with an accidental nod to Lucy Lawless. The science section covers Eris's 27% mass advantage over Pluto and its slightly smaller diameter, the 2010 stellar occultation, the 558-year orbit tilted 44 degrees and stretching to 98 AU, the scattered disc, its brilliant white methane frost, a day locked to Dysnomia's 15.8-day orbit, hints of a buried ocean and the New Horizons glimpse from inside the Kuiper belt. It ends with the myth: the uninvited goddess, the golden apple 'for the fairest' and the Judgement of Paris that started the Trojan War. Difficulty runs from easy to expert and is shown on each question, so it suits a classroom, a pub quiz or a space enthusiast. Every answer was checked against a reference page and each question carries its citation. Try our Pluto, Planets and Solar System quizzes for more.
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Q 01Eris holds which record among the dwarf planets?
Most massive
It outweighs Pluto by about 27%, though Pluto is a hair wider.
Q 02Who led the team that discovered Eris?
Mike Brown
Chad Trujillo and David Rabinowitz were his co-discoverers; Brown later wrote a book called How I Killed Pluto.
Q 03In what year was Eris discovered on archived images?
2005
The images had been taken in October 2003 but its slow motion had been filtered out by the search software.
Q 04Which observatory's Samuel Oschin telescope took the images in which Eris was found?
Palomar
The 1.2-metre Schmidt camera in California had already found Quaoar, Orcus and Sedna for the same team.
Q 05Why did the search software initially miss Eris in the 2003 images?
It moved too slowly across the sky
The code ignored anything slower than 1.5 arcseconds per hour; Sedna's discovery prompted a re-check by eye.
Q 06Because it seemed bigger than Pluto, NASA initially described Eris as what?
The tenth planet
That claim forced the IAU to define 'planet' for the first time in 2006, and both bodies became dwarf planets.
Q 07What nickname did the discovery team use for Eris before it was formally named?
Xena
They had saved it for the first body they found bigger than Pluto; the moon became 'Gabrielle' after the sidekick.
Q 08Which TV series inspired the informal nickname for Eris?
Xena: Warrior Princess
Brown said they liked that it started with an X, as in Planet X, and that it 'sounds mythological'.
Q 09Eris is named after the Greek goddess of what?
Strife and discord
Brown said the object had 'caused strife and discord' too, by triggering the fight over Pluto's status.
Q 10What was Eris's provisional designation before it got its name?
2003 UB313
It kept the awkward label for an unusually long time while the IAU decided what a planet was.
Q 11Which name, winner of a New Scientist poll, could not be used because an asteroid already had it?
Persephone
399 Persephone had taken it long before; 'Xena' came fourth in the same poll.
Q 12Brown initially wanted to call the object 'Lila', a Hindu concept pronounced like whose name?
His newborn daughter Lilah
He had put the name in a web address, then left it there by accident during the Haumea controversy.
Q 13What is the name of Eris's confirmed moon?
Dysnomia
She was the goddess of lawlessness and daughter of Eris in Greek myth.
Q 21How long does Eris take to orbit the Sun?
558 years
It reached aphelion around 1977 and will not be at perihelion again until December 2257.
Q 22Eris's orbit ranges from 38 AU in to how far out from the Sun?
98 AU
At perihelion it actually comes inside Pluto's orbit, but it never nears Neptune.
Q 23Eris's orbit is tilted to the plane of the planets by roughly how much?
About 44°
That steep tilt is why nobody found it earlier: surveys concentrate on the ecliptic.
Q 24Eris belongs to which population of trans-Neptunian objects?
Q 14Eris's moon is named after a daughter of Eris who was the goddess of what?
Lawlessness
Brown also liked its similarity to his wife's name, Diane; and Xena was played by Lucy Lawless.
Q 15The moon's name carries an accidental nod to which actress, who played Xena?
Lucy Lawless
'Dysnomia' means lawlessness; Brown insists the pun was unintentional.
Q 16What did the discovery of the moon allow astronomers to calculate for the first time?
The dwarf planet's mass
It came out 27% greater than Pluto's, settling the mass argument for good.
Q 17Eris is about how much more massive than Pluto?
27%
Its density of 2.52 g/cm3 shows it must be mostly rock.
Q 18What is Eris's measured diameter?
About 2,326 km
A November 2010 stellar occultation showed it was slightly smaller than Pluto after all.
Q 19How was Eris's true size finally pinned down in 2010?
By watching it pass in front of a star
It was one of the most distant stellar occultations ever observed from Earth.
Q 20Which 2015 mission confirmed that Eris's old rival is slightly larger in volume after all?
New Horizons at Pluto
For nearly a decade Eris had been thought the ninth-largest object orbiting the Sun.
The scattered disc
Its eccentric, inclined orbit is the signature of a body flung outward by Neptune long ago.
Q 25In February 2016, how far was Eris from the Sun?
Some 96 AU
More than three times Neptune's distance; it was the most distant known natural object until 2018.
Q 26Eris's surface is unusual for a distant icy world in what way?
It is almost pure white and uniform
Its albedo of 0.96 makes it one of the most reflective bodies known, probably from freshly condensed methane frost.
Q 27Which ice did spectroscopy at Gemini North reveal on Eris's surface in 2005?
Methane
That made it similar to Pluto and Triton, then the only distant bodies known to have surface methane.
Q 28Roughly what is Eris's surface temperature range?
About −243 °C to −217 °C
When it swings closer to the Sun some of its ices may sublime into a temporary atmosphere.
Q 29Eris and its moon form the second known example of what kind of system, after Pluto and Charon?
Doubly tidally locked (double-synchronous)
Eris's day equals Dysnomia's 15.78-day orbit, and each always shows the other the same face.
Q 30How long is a day on Eris?
About 15.8 Earth days
It is locked to its moon's orbit; earlier estimates ranged from tens of hours to several days.